The invention relates to an electronic system for controlling the inking of a printing press and, more particularly, such a system wherein a sheet printed by the printing press is measured photoelectrically in a number of test areas, the measured values obtained thereby are processed in conjunction with setpoint values to form control data, and the inking of the printing press is controlled in accordance with the control data.
Control of inking in the course of a printing process represents the most important possibility for influencing the inking and, thereby, the impression of the image. The aim of inking control is to achieve as good a color agreement as possible between an original and a printed image in a production run. The control of inking in accordance with colorimetric quantities is regarded as a considerable improvement in this connection, because feedback control, in the sense of matching the setpoint values and actual values of a color locus, corresponds to a good approximation to color perception by the human eye.
Spectral measurements of the diffuse reflection of color-measuring fields, the mathematical conversion of these measured values into colorimetric quantities and further into control data for adjusting the inking elements of a printing press have become known hererofore. In European Published Non-Prosecuted Application (EP-OS) 0 228 347, there is disclosed, in addition to a suitably equipped printing press and a measuring device for such a printing press, a method of controlling the inking of a printing press. For color matching an original and a printed image in a production run, the spectral diffuse reflection is measured in color-measuring fields which are also printed by the printing press, and corresponding color coordinates are determined therefrom. By a comparison with the setpoint diffuse reflection and the setpoint color coordinates, respectively, the distance between the setpoint and actual color loci is calculated and is converted into changes in the thicknesses of the layers of the printing inks. The inking elements are controlled in accordance with these calculated changes of the layer thicknesses of the individual printing inks in such a manner that the total distance between the actual and setpoint color loci in the corresponding color space is minimized.
In the published non-prosecuted patent application 89 100 150.6 of the People's Republic of China, there is likewise proposed a process for controlling the inking of a printing press on a colorimetric basis, the process being distinguished by its high speed of convergence, i.e. its rapid detection of the relative distance between the actual and setpoint color loci.
Although colorimetric control conforms with the color perception of the human eye, the quality of inking control in both processes is limited by the fact that no attention is paid to technical process-related limits inherent in offset printing. Once calculated, a setpoint color locus remains the setpoint color locus throughout the entire printing process and is triggered or selected even if this involves the exceeding of or not reaching, respectively, a maximum or minimum allowable layer thicknesses of one or more printing inks. By the maximum or minimum allowable layer thickness there is meant, in this connection, the layer thickness at which the accompanying increase in the size of the half-tone dots of a printing ink leads to impermisible tonal values. These changes in the tonal-value increase of the printed image in a production run lead to shifting of the color locus, which is reflected in color distortions which are not tolerable, particularly in the range of critical shades of color (e.g. skin shades). German Published Non-Prosecuted Patent Application (DE-OS) 38 12 099.2 proposes a process which, based upon colorimetric control and taking into account the parameter of a maximum thickness of the layer of individual printing inks, permits a considerably more reliable and, thereby, extensively automatic control of the inking of a printing press.
If, when triggered or selected directly, the setpoint color locus can be attained only if this were to involve exceeding the maximum thickness of a printing-ink layer, then, proceeding from this color locus at the maximum thickness of the printing-ink layer, tests are made as to whether, by changing the thicknesses of the layers of other printing inks, it is possible to obtain a color locus that lies within the specified tolerances about the setpoint color locus. If this were to yield a color locus which involve the exceeding of the maximum thickness of a second printing-ink layer, then, proceeding from this color locus, an attempt is made, by changing the remaining printing ink, once again to find a color locus which lies within the specified tolerances about the setpoint color locus. As soon as a color locus is found, by means of the successive process steps, which lies within the maximum allowable layer thicknesses of all of the printing inks involved in the printing job, this color locus is triggered or selected as the new setpoint color locus. If no success is had in obtaining a color locus within the tolerance range about the setpoint color locus without exceeding the maximum layer thickness of at least one printing ink, provision is made for performing a manual intervention into the control process. Armed with his expert knowledge, the printer is able to decide whether to extend the tolerance range about the setpoint color locus or whether to increase the maximum layer thickness of the critical printing ink. Once the appropriate decision has been made, the afore-described process starts afresh and, subject to the parameter of maximum layer thicknesses, again seeks for a color locus situated as close as possible to the setpoint color locus.
Although this method of inking control takes into account the fact that, in order to achieve optimum agreement between original and printed image in a production run, it is not permissible to exceed the maximum layer thicknesses of the individual printing inks, it is disadvantageous, in this connection, to assume that the selected maximum layer thickness is to be constant throughout the entire printing process. No account is taken of process-dependent changes in the maximum layer thickness, for example, due to changes in the consistency of the printing inks under the influence of temperature or humidity, due to soiling of the rubber blanket or due to changes in the paper.
Moreover, it is more advantageous for the optimum control of inking if the tonal-value increase is taken into account directly and not indirectly, i.e. via the maximum/minimum layer thickness. The tonal-value increase, defined as the difference of the screen tonal values (percentage of optically active area coverages) of screened film and screen print, is of decisive importance for the color impression of a printed image in a production run, as noted hereinbefore. Even minor changes in the area coverage of the screen of only one color lead to non-tolerable color distortions in the case of high-grade printed products.
With regard to the tonal-value increase, the German Published Non-Prosecuted Application 38 12 099.2 merely provides that, if a specified tolerance range of the tonal-value increase is exceeded, a warning signal is given; the inking control itself is not thereby influenced. In order, however, to achieve optimum print quality and to obtain extensively automatic control of inking, it is not permissible to exceed maximum allowable changes in the tonal-value increase or to exceed limits, derived therefrom, for the maximum allowable layer-thickness tolerances and, in the color space, for the color tolerances, respectively.